1. Field of the invention
The present invention concerns a process for heating a body formed of a heat-hardenable substance by means of electromagnetic radiation, in order to polymerize it.
2. Description of the prior art
There are known heating processes employing a cavity in which the substance or object to be heated is placed to be exposed to electromagnetic radiation. This type of process employs a device, commonly known as a "microwave oven", which finds applications in industrial vulcanization and polymerization processes involving heating certain substances.
The electromagnetic field alternates at a very high frequency (2.45 GHz according to French regulations, for example) and causes dielectric losses which heat the body and thus achieve the required result.
However, because of multiple reflections of the waves from the walls of the oven, it is difficult to control the spatial distribution of the energy transmitted to the substance.
For this reason, the disposition in the oven of any conductive material (especially metal) part which would disturb the direction of the electromagnetic field through acting as a reflecting screen is depreciated.
The lack of control over the orientation of the electromagnetic field produces irregularities in the field causing irregular heating within the substance, certain areas being more heated than others. It is known that an increase in the temperature of a substance generally increases its loss factor.
By virtue of this increase, the loss factor conditions the temperature rise. As this is directly proportional to the losses, the process rapidly becomes locally unstable and the regions of the substance subject to the highest temperature rises absorb the greater part of the electromagnetic energy.
Apart from the fact that other regions are not sufficiently heated, the regions in which these instabilities (known as "thermal runaway") occur are overheated and therefore often burnt.
The importance of the consequences described naturally varies depending on the nature of the operation to be carried out, and it will be obvious that it becomes critical in operations to vulcanize or polymerize a synthetic material which breaks down at relatively low temperatures.
The invention is directed towards remedying the disadvantages due to lack of control over the direction of the electromagnetic field to which the substance to be heated is exposed, by appropriately guiding the field.
In this way it is possible to control the distribution of energy within the substance and to bring about a homogeneous increase in the temperature of the substance.